Optical compensators are used in prior art to improve the optical properties of liquid crystal displays (LCD), such as the contrast ratio and the grey scale representation at large viewing angles. For example in uncompensated displays of the TN or STN type at large viewing angles often a change of the grey levels and even grey scale inversion, as well as a loss of contrast and undesired changes of the colour gamut are observed.
An overview of the LCD technology and the principles and methods of optical compensation of LCDs is given in U.S. Pat. No. 5,619,352, the entire disclosure of which is incorporated into this application by way of reference. As described in U.S. Pat. No. 5,619,352, to improve the contrast of a display at wide viewing angles a negatively birefringent C-plate compensator can be used, however, such a compensator does not improve the greyscale representation of the display. On the other hand, to suppress or even eliminate grey scale inversion and improve the grey scale stability U.S. Pat. No. 5,619,352 suggests to use a birefringent O-plate compensator. An O-plate compensator as described in U.S. Pat. No. 5,619,352 includes an O-plate, and may additionally include one or more A-plates and/or negative C-plates.
The terms ‘O-plate’, ‘A-plate’ and ‘C-plate’ as used in U.S. Pat. No. 5,619,352 and throughout this invention have the following meanings. An ‘O-plate’ is an optical retarder utilizing a layer of a positively birefringent (e.g. liquid crystal) material with its principal optical axis oriented at an oblique angle with respect to the plane of the layer. An ‘A-plate’ is an optical retarder utilizing a layer of uniaxially birefringent material with its extraordinary axis oriented parallel to the plane of the layer, and its ordinary axis (also called ‘a-axis’) oriented perpendicular to the plane of the layer, i.e. parallel to the direction of normally incident light. A ‘C-plate’ is an optical retarder utilizing a layer of a uniaxially birefringent material with its extraordinary axis (also called ‘c-axis’) perpendicular to the plane of the layer, i.e. parallel to the direction of normally incident light.
Negative birefringent C-plate retarders in prior art have been prepared for example from uniaxially compressed films of isotropic polymers, by vapour deposition of inorganic thin films, as described for example in U.S. Pat. No. 5,196,953, or from negatively birefringent liquid crystal materials. However, stretched or compressed polymer films often show only moderate birefringence and require high film thickness, vapour deposition requires complicated manufacturing procedures, and negatively birefringent liquid crystal materials are often less easily available and more expensive than positively birefringent materials.
To overcome these disadvantages, it has recently been suggested, for example in WO 01/20393 and WO 01/20394, to use a cholesteric liquid crystal film with short pitch, typically with its Bragg reflection band in the UV region of the electromagnetic spectrum. Such a film exhibits negative birefringent C-type retardation for wavelengths greater than its reflection maximum. The refractive index ellipsoid of this type of film approximates to that of a vertically aligned liquid crystal with negative birefringence. Such a retardation film can be used for example to cancel off-axis retardation in the homeotropically driven dark state of a TN-LCD, and thus significantly improve the viewing angle of the LC display.
WO 01/20393 discloses a compensator that is a combination of a planar A-plate, an O-plate and a negative C-plate, wherein the negative C-plate comprises a short-pitch cholesteric LC film. When used for example in a TN-LCD, this combination provides excellent contrast at horizontal viewing angles and reduces unwanted changes of the colour gamut. However, its performance at vertical viewing angles is limited. Furthermore, the use of multiple retardation films is expensive and raises manufacturing and durability problems.
One aim of the present invention is to provide an optical compensator which has improved performance for compensation of LCDs, is easy to manufacture, in particularly for mass production, and does not have the drawbacks of prior art compensators as described above. Another aim of the invention is to provide an improved method for its preparation. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors have found that the above described problems can be solved, and an optical compensator with superior performance can be obtained, by combining multiple films in a single layer and by using a biaxial C-plate retarder. It was found that a biaxial negative C-plate retarder in its optical properties approximates to a combination of a planar A-plate and a negative C-plate, but shows better optical performance than such a combination. The in-plane anisotropy of the biaxial negative C-plate retarder (Δnxy) approximates to the A-plate and the out-of-plane anisotropy (Δnxz and Δnyz) to the negative C-plate. Simulations have shown that the optical performance of the biaxial negative C-plate retarder is surprisingly superior to that of the A-plate and negative C-plate stacked sequentially, and shows exceptionally good viewing-angle performance for liquid crystal displays. Furthermore, the use of a single biaxial film instead of two stacked films reduces costs and manufacturing problems.
Moreover, the inventors have found a new and improved method of preparing such a film, by using a polymerisable chiral liquid crystal material comprising a photosensitive compound and a dichroic photoinitiator, and irradiating the material with polarised UV light, leading to a change of shape or structure and thus a birefringence change of the photosensitive compounds in some parts of the film, which is then frozen in by polymerisation.
WO 03/01544 discloses a cholesteric biaxial film having a deformed helix structure with an elliptical refractive index and a method of preparing such a film from a polymerisable cholesteric material comprising a dichroic photoinitiator. However, it does not disclose a biaxial film with varying birefringence obtained by the methods and from materials comprising a photosensitive compound as claimed in the present invention.
U.S. Pat. No. 6,685,998 discloses a compensator comprising a transparent substrate and a film formed from rod-like liquid crystal molecules, wherein said film has three different principal refractive indices and the liquid crystal molecules are oriented in cholesteric alignment. However, it does not disclose a biaxial cholesteric film reflecting in the UV range and having a periodically varying birefringence obtained by the methods and from the materials as claimed in the present invention.
Definition of Terms
The terms ‘photoreactive’, ‘photosensitive’ and ‘photoreaction’ refer to compounds which change their structure or shape upon photoirradiation by reactions including, but not limited to, photoisomerisation, photo-induced 2+2 cycloaddition, photo-fries arrangement or a comparable photodegradation process. Photopolymerisation reactions are not included in these meanings. However, the photoreactive or photosensitive compounds as described in this invention can in addition also be polymerisable or photopolymerisable.
The term ‘film’ includes rigid or flexible, self-supporting or free-standing films with mechanical stability, as well as coatings or layers on a supporting substrate or between two substrates.
The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal or mesogenic compound’ means materials or compounds comprising one or more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e. groups with the ability to induce liquid crystal (LC) phase behaviour. LC compounds with rod-shaped or board-shaped groups are also known in the art as ‘calamitic’ liquid crystals. LC compounds with a disk-shaped group are also known in the art as ‘discotic’ liquid crystals. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit an LC phase themselves. It is also possible that they show LC phase behaviour only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerised.
For the sake of simplicity, the term ‘liquid crystal material’ is used hereinafter for both mesogenic and LC materials.
Polymerisable compounds with one polymerisable group are also referred to as ‘monoreactive’ compounds, compounds with two polymerisable groups as ‘direactive’ compounds, and compounds with more than two polymerisable groups as ‘multireactive’ compounds. Compounds without a polymerisable group are also referred to as ‘non-reactive’ compounds.
The term ‘reactive mesogen’ (RM) means a polymerisable mesogenic or liquid crystal compound.
The term ‘director’ is known in prior art and means the preferred orientation direction of the long molecular axes (in case of calamitic compounds) or short molecular axis (in case of discotic compounds) of the mesogenic groups in an LC material.
In films comprising uniaxially positive birefringent LC material the optical axis is given by the director.
The term ‘cholesteric structure’ or ‘helically twisted structure’ refers to a film comprising LC molecules wherein the director is parallel to the film plane and is helically twisted around an axis perpendicular to the film plane.
The term ‘homeotropic structure’ or ‘homeotropic orientation’ refers to a film wherein the optical axis is substantially perpendicular to the film plane.
The term ‘planar structure’ or ‘planar orientation’ refers to a film wherein the optical axis is substantially parallel to the film plane.
The term ‘tilted structure’ or ‘tilted orientation’ refers to a film wherein the optical axis is tilted at an angle θ between 0 and 90° relative to the film plane.
The term ‘splayed structure’ or ‘splayed orientation’ means a tilted orientation as defined above, wherein the tilt angle varies in the direction perpendicular to the film plane, preferably from a minimum to a maximum value.
The average tilt angle θave is defined as follows
      θ    ave    =                    ∑                              d            ′                    =          0                d            ⁢                        θ          ′                ⁡                  (                      d            ′                    )                      d  wherein θ′(d′) is the local tilt angle at the thickness d′ within the film, and d is the total thickness of the film.
The tilt angle of a splayed film hereinafter is given as the average tilt angle θave, unless stated otherwise.
For sake of simplicity, an optical film with twisted, planar, homeotropic, tilted or splayed orientation or structure is hereinafter also referred to as ‘twisted film’, ‘planar film’, ‘homeotropic film’, ‘tilted film’ or ‘splayed film’, respectively.
Tilted and splayed films are also referred to as ‘O plate’. A planar film is also referred to as ‘A plate’ or ‘planar A plate’.
“E-mode” refers to a twisted nematic liquid crystal display (TN-LCD) where the input polarisation direction is substantially parallel to the director of the LC molecules when entering the display cell, i.e. along the extraordinary (E) refractive index. “O-mode” refers to a TN-LCD where the input polarisation is substantially perpendicular to the director when entering the display cell, i.e. along the ordinary (O) refractive index.